Proliferation of normal cells involves orderly occurrence of cell division and its pause which proceed along the cell cycle, whereas proliferation of cancer cells is characterized by its disorderliness. Therefore, it is presumed that abnormality in the cell cycle control mechanism is directly related to oncogenesis or malignant alteration of cancer. The cell cycle of mammal cells is controlled by serine/threonine kinases that are generically called the cyclin dependent kinases (hereinafter, referred to as “Cdk”) family, and in order for a Cdk to express the enzyme activity, the Cdk is required to form a complex with a regulatory subunit called cyclin. Cyclins also constitute a family, and it is believed that each Cdk molecule regulates the progression of certain phases in the cell cycle by forming a complex with a limited type of cyclin molecule which is specifically expressed at the corresponding phase of the cell cycle. For example, D-type cyclins bind with Cdk4 or Cdk6 to regulate the progression of G1 phase; cyclin E-Cdk2 regulates the G1/S boundary; cyclin A-Cdk2 regulates the progression of S phase; and cyclin B-cdc2 regulates the progression of G2/M. In addition, D-type cyclins are known to have three subtypes of D1, D2 and D3, and the activity of Cdk is believed to be controlled not only by the binding with cyclins, but also by phosphorylation/dephosphorylation of Cdk molecules, decomposition of cyclin molecules, and binding with Cdk inhibitory proteins [Advance Cancer Res., Vol. 66, 181-212 (1995); Current Opin. Cell Biol., Vol. 7, 773-780 (1995); and Nature, Vol. 374, 131-134 (1995)].
The Cdk inhibitory proteins found in mammal cells are classified into two major classes of Cip/Kip family and INK4 family, on the basis of the differences in structure and nature. The former widely inhibits cyclin-Cdk complexes, whereas the latter binds with Cdk4 and Cdk6 to specifically inhibit them [Nature, Vol. 366, 704-707 (1993); Mol. Cell. Biol., Vol. 15, 2627-2681 (1995); and Genes Dev., Vol. 9, 1149-1163 (1995)].
A representative example of the former may be p21 (Sdi1/Cip1/Waf1), whose RNA transcription is induced by p53, a product of tumor suppressor gene [Genes Dev., Vol. 9, 935-944 (1995)].
On the other hand, for example, p16(INK4a/MTS1/CDK4I/CDKN2) is one of the Cdk inhibitory proteins belonging to the latter family. p16 gene is found at human chromosome region 9p21, which is very frequently found with anomalies in human cancer cells, and in fact, deletion of p16 gene has been reported in a number of cases in the clinical practice. It has been also reported that the frequency of cancer occurrence is high in p16-knockout mice [Nature Genet., Vol. 8, 27-32 (1994); Trends Genet., Vol. 11, 136-140 (1995); and Cell, Vol. 85, 27-37 (1996)].
Each Cdk controls the progression of cell cycle by phosphorylating a target protein found at a specific phase of the cell cycle, but among such target proteins, retinoblastoma (RB) protein is considered to be one of the most important target proteins. The RB protein is a key protein in the progression from G1 phase to S phase, and is rapidly phosphorylated during the term from late G1 phase to initial S phase. This phosphorylation is believed to be carried out by cyclin D-Cdk4/Cdk6 complex and then by cyclin E-Cdk2 complex, which are associated with the progression of cell cycle. When RB protein is hyperphosphorylated, a complex that has been formed by a hypophosphorylated form of RB and a transcription factor E2F until that time point at early G1 phase, dissociates. As a result, E2F becomes a transcriptional activator, and at the same time, the suppression of the promoter activity by the RB-E2F complex is removed, thereby E2F-dependent transcription being activated. Currently, a Cdk-RB pathway involving E2F and its inhibitor RB protein, and Cdk4/Cdk6 regulating the function of RB protein in a suppressive manner, Cdk inhibitory protein regulating the kinase activity thereof, and D-type cyclins, is construed as an important mechanism controlling the progression from G1 phase to S phase [Cell, Vol. 58, 1097-1105 (1989); Cell, Vol. 65, 1053-1061 (1991); Oncogene, Vol. 7, 1067-1074 (1992); Current Opin. Cell Biol., Vol. 8, 805-814 (1996); and Mol. Cell. Biol., Vol. 18, 753-761 (1998)]. In fact, the E2F-binding DNA sequence is, for example, located in the upstream of many cell proliferation-related genes that are important in S phase, and it is reported that in a plurality of such genes among them, transcription is activated in an E2F-dependent manner over a period spanning from late G1 phase to early S phase [EMBO J., Vol. 9, 2179-2184 (1990); and Mol. Cell. Biol., Vol. 13, 1610-1618 (1993)].
Abnormalities in several factors constituting the Cdk-RB pathway, for example, deletion of functional p16, high expression of cyclin D1 or high expression of Cdk4, deletion of functional RB protein, or the like, have been very frequently detected in human cancer [Science, Vol. 254, 1138-1146 (1991); Cancer Res., Vol. 53, 5535-5541 (1993); and Current Opin. Cell Biol., Vol. 8, 805-814 (1996)]. These are all abnormalities in the direction of promoting the progression from G1 phase to S phase, and it is obvious that this pathway is playing an important role in canceration or abnormal proliferation of cancer cells.
The Applicant of the present invention created unique compounds having a Cdk inhibitory effect in the past, and have filed patent applications concerning novel biaryl urea derivatives (WO 01/07411), novel pyrazinone derivatives (WO 02/002550), and novel quinoxalinone derivatives (WO 04/039809).
However, reports on aminothiazole derivatives having excellent selective inhibitory effects against Cdk4 and/or Cdk6 cannot be found so far (WO 01/72745). A compound having an excellent selective inhibitory effect against Cdk4 and/or Cdk6 in contrast to other Cdks, is expected to serve as an anticancer agent with a greater margin of safety.
Furthermore, since Cdk4 and Cdk6 are factors generally related to the control of cell cycle and cell proliferation, a selective inhibitor thereof is expected to be beneficial for the treatment of diseases inducing abnormalities in the cell cycle and cell proliferation, which include for example, but are not limited to, arthritis, arteriosclerosis, pulmonary fibrosis and cerebral infarction.
In these cases, it is anticipated that suppression of cell cycle and cell proliferation through Cdk inhibition will be effective, based on the following technical expertise.
In the case of rheumatoid arthritis, over-proliferation of synovial tissue in the affected area is well known. Proliferation of cells from this tissue is relevant to the level of expression of Cdk inhibitory proteins, p21 and p16, and it is reported that when p16 is introduced onto the affected areas of rheumatoid arthritis-model animals, there is an improvement in the symptoms [Nat. Med., Vol. 5, 760-767 (1999)].
In the case of arteriosclerosis, over-proliferation of smooth muscle cells in the endothelial lining of arterial walls is important, but it is known that suppression of Cdk expression by antisense oligonucleotides in an experimental plaque model using a balloon catheter, and forced expression of p21 and p27 by adenovirus vectors, inhibit neointima formation [Int. J. Mol. Med., Vol. 2, 81-89 (1998)].
It is also reported that expression of cell cycle inhibitory protein p21 induced by adenovirus vectors is effective in pulmonary fibrosis-model mice [Am. J. Physiol. Lung. Cell Mol. Physiol., Vol. 286, L727-L733 (2004)].
In a rat cerebral infarction model, it is known that neuronal death due to localized ischemia enhances the cyclin D1/Cdk4 level, and it is reported that neuronal death is suppressed by administration of a non-selective Cdk inhibitor, flavopyridol [Proc. Natl. Acad. Sci. USA, Vol. 97, 10254-10259 (2000)].